1. Field of the Invention
The present invention relates to an encryption communication method applied when encryption communication is established between a calling terminal device and a called terminal device, and to a terminal device for carrying out the encryption communication method.
2. Description of the Prior Art
In the field of communication, communication data has been conventionally subjected to encryption processing from the point of view of security protection of the communication data. For example, in facsimile communication for transmitting and receiving information corresponding to a document, the information which is communication data has been subjected to encryption processing so as to protect the security of the information.
The encryption processing of the information is performed on the basis of a so-called encryption key. Specifically, the information is subjected to a predetermined logical operation using the encryption key, thereby producing encrypted information entirely different from the original information. On the other hand, decryption processing for restoring the encrypted information into the original information is achieved by subjecting the encrypted information to a predetermined logical operation using the encryption key. The logical operation for the decryption processing is an operation opposite to the logical operation for the encryption processing.
The encryption processing and the decryption processing thus require a common encryption key. Therefore, a method in which an encryption key is previously determined between the transmission side and the receiving side of the encryption communication and the determined encryption key is previously registered in nonvolatile memories respectively provided in a terminal device on the transmission side and a terminal device on the receiving side has been conventionally adopted.
In the above described method, however, preparation work for establishing encryption communication is complicated because the encryption key must be determined in advance and registered in the nonvolatile memories.
Furthermore, when the encryption key stored in the nonvolatile memories disappears or is known by a third person for some reason, the above described preparation work must be performed again.
In order to solve the above described problems, therefore, it is considered that the encryption key itself is transmitted. In a case where the encryption key itself is transmitted, however, the danger of leakage of the contents of the communication to the third person is increased if the encryption key is monitored on the communication line.